1. Field of the Invention
The present invention relates to a type of image display device and a driving method thereof, and particularly relates to the image display device and drive method thereof which is capable of adjusting brightness by use of a simple circuit structure and by a simple operation.
2. Background Art
Recently, the demand for display devices has increased notably, and particular attention is being paid to flat displays represented by liquid crystal displays (LCD) and plasma displays (PD).
In particular, light emitting type display devices such as electro-luminescence display devices are advantageous in high visibility, wide angle of visibility, and they have the feature that they do not need the back light required for the LCD devices. Display devices using organic electro-luminescent (EL) elements are attracting attention as flat display devices with high response characteristics.
There are two driving systems for driving the dot matrix display devices having organic EL elements, in which one is a simple matrix system and the other is an active matrix system.
FIG. 10 is a block diagram showing an organic EL display using organic EL elements. This organic EL device is constituted by a color organic EL display 101 in a QVGA class using NTSC signals, a column driving circuit 102 for driving the column side of the display, and a row driving circuit 103 for driving the row side of the display.
This color organic EL display panel 101 has a matrix structure in which a plurality of lines of transparent anode electrodes (data electrodes), an organic EL thin film, and a plurality of lines of transparent cathode electrodes (scanning electrodes) are sequentially formed on a transparent plate such as a glass plate such that the anode electrode lines and the cathode electrode lines cross at a right angle to each other.
FIG. 11 is a timing chart showing the operational timing of a color organic EL display device. This display device is driven by a single scan drive method and comprises 240 lines of cathode electrodes (scanning electrodes) and 320×3 (RGB)=960 lines of anode electrodes (data electrodes).
The duty factor of this color organic EL display device becomes 1/240, since the cathode electrodes of this color organic EL display device are sequentially driven by the row driving circuit 103 (the scanning electrode), and since each of the 240 scanning electrode lines Y1 to Y240 are scanned in sequence to form a single image plane. In this driving device, one scanning line is always used for scanning at one time, so that this drive method is called a single scanning drive system.
In contrast to this single scanning drive system, there is another driving system called a double scanning drive system.
This double scanning drive system is a drive method in which two scanning electrode lines at the row side are always scanned in order to increase the brightness of the display. In the case of a QVGA class color organic EL display, horizontal scanning electrode lines are divided into two upper and lower groups (each group has 120 lines) by a horizontal location and two scanning electrode lines out of each group are driven by the single scanning drive system to form a single image so as to change the duty factor to 1/120. This double scanning drive system is disclosed, for example, in Japanese Unexamined Patent Application, First Publication No. Sho 61-264876.
However, the problem arises in the above-described conventional simple matrix system that the brightness of the organic EL device is reduced as the number of scanning electrode lines is increased and as the duty factor is reduced since the light emitting time is reduced in the above cases.
The brightness of the organic EL element is proportional to a current density applied to an emitting pixel. Thus, one of the measures to increase the brightness of the organic EL element is to increase the current density by increasing the voltage applied to the organic EL element.
However, the problem arises that application of a high voltage to the organic EL element reduces its service life. In addition, it becomes necessary to provide a circuit for regulating the voltage for every scanning electrodes or for every data electrode and another problem arises in that the constitution and control of the circuit becomes complicated, which results in increasing the cost of such a product.
In the case of, for example, the single scan drive system, the brightness of the organic EL display device is reduced, since the duty factor is reduced as the number of scanning electrodes increases because each light emitting element is turned on one by one by driving each scanning electrode. In the case of, for example, a QVGA class organic EL display, the number of scanning electrodes is 240, and the duty factor is 1/240, and the maximum brightness of the display is 70 cd/m2, which is insufficient brightness for a practical display device.
The brightness of a color EL display can be improved in the case of the above-described double scan drive system. However, it becomes necessary for the organic EL display device to provide a memory device at the column side and the RGB signal fine adjustment circuit for according amplification levels of RGB signals becomes complicated, which results in increasing the cost of the product.